What is the actual purpose of AC power?


I have a very limited knowledge of electrical engineering that almost entirely revolves around music equipment. From everything that I’ve seen, music equipment ends up converting the AC power to DC power right at the power entry point (or at least soon after). It appears they’re converting AC power to a higher voltage and current DC power. Is the only real advantage of AC power to pump more power over a line to where devices will then manipulate it to what they need in DC? Are there any common household items that actually fully operate off of AC power or does everything convert to DC at some point?

In: 5

It was chosen because it’s easy to multiply and divide the voltage with transformers. The voltage can be raised very high to transmit over long distances with minimal loss to cable resistance.. then dropped back down for distribution in neighborhoods, then again to feed homes.

Most light bulbs including florescents use a.c.
(Led bulbs have a converter inside)

I believe that if you go way back in history, it was determined that it was easier to transmit a.c. over greater distances than d.c.

To give you the simplest possible answer, it is much easier to transmit A/C over long distances.

So, if you want to create a national power grid to supply a whole country then A/C is the way.

If you used DC then you would need boosters all over the place to keep the signal strong and useable.

This is an excellent question. In the early days of technology, it was not obvious which one would be the best choice for electrical distribution. Google “Edison v Westinghouse” for a list of historical works describing what went into this. It’s fascinating. Spoiler alert: as mentioned in all the other answers to this question, AC won because it’s far easier to manipulate from an engineering perspective. For long-distance transmission, you want low current and (very) high voltage. End users, on the other hand, want high current and (relatively) low voltage. AC makes it easy to convert from one to the other using transformers.

I thought ac was supposed to be safer. Did I dream that?

Household items that will likely operate off AC:

* Anything with big motors – fans, washer and dryer, leaf blower, power tools, etc.; it’s simple enough to design motors to operate with AC magnetic fields inside the motor, rather than DC magnetic fields.

* Heaters – any sort of power (AC or DC) passing through a thin wire will heat it up, no need to convert to get heat out of electricity.

Household items that will very likely operate off DC:

* Electronics in general, and audio equipment in particular, the AC frequencies can introduce unwanted “noise” into the circuitry, so DC must be used. Power line AC has a frequency of 50 or 60 Hz (depending on country), and humans can hear as low as 16-20 Hz so for audio equipment it literally is noise that must be filtered out.

* Sensors usually convert temperature to voltage, weight to voltage, humidity to voltage, etc., so DC is a requirement for the sensor to function. And also, typically the sensors are attached to electronics that interprets the sensor data, and the electronics typically use DC anyway.

Imagine a pole with a saw blade on the end.

AC power is sorta like pulling it back and forth.

DC power would be like having a massive saw and pushing it continuously.

But ac power is used because you can change voltage with a transformer with no moving parts and minimal points of failure.

Transformers are basically just iron plates held near each other without touching. The AC current causes a magnetic field and even though the line and load don’t touch each other this creates a power in the load side of a different voltage.

This is how we get 110v or 230v from our homes despite power plants making stuff like 10000 volts for the main lines.

As people have said (roughly): you can induce voltage into a wire by moving a wire through a magnetic field. (the wire must be “cutting” magnetic flux lines). Constant current creates a steady magnetic field so voltage in a transformer, for example, is only induced when power is turned on or off (this is how the coil to power the spark plugs in a car work). Since ac power is constantly changing current its magnetic field is constantly building and collapsung and thus the magnetic field lines are constantly crossing the wires in a transformer. Since you need really high voltage to transmit power efficienciently (or even to be possible for long distances) and much lower voltages to not electrocute anything within a few feet of an outlet, you need to have an easy way to change voltage efficiently, something only really possible with transformers and thus ac power. Furthermore, on the most basic level, every rotating style generator (so only excluding things like solar or batteries) creates ac power. (yes there are dc generators but they use diode packs to rectify ac into DC) finally, as people said, big (ish) motors, many lights, stoves, pretty much any heater, etc use ac directly. Last point of clarification: as power is not any more easily transmissible than DC for the same voltage/current, ac simply lends itself to easily changing voltage. Also, it is easy to convert ac to DC (I could build a rectifier with 6 diodes and ideally a capacitor super easily), but pretty hard actually to convert DC to ac with any efficiency.

Back in the day, it was incredibly difficult to transmit DC power long distances. To power a city you’d basically have to install a generator every block.

AC, otoh, was relatively easy to transmit long distances, so you could (for example) power NYC with a hydroelectric plant in Niagra Falls.

These days we do know how to transmit DC long distances and it is more effecient than AC, but replacing the entire power grid for any country is a multi billion if not multi trillion dollar project

Electric generators and motors can use dc or ac, not both.

DC generators and motors need brushes to catch the power from the spinning part, and this brushes require big big maintenance. When AC was discovered, it allowed to make generators and motors at 2/3 of the price of a DC one, and they are completely maintenance free, because they don’t need a brush system. So they won, and were adopted in mass.

Not having brushes allows to produce higher voltages without issues. Higher voltage allows to transfer the same power using thinner wires, this makes transport and the generator parts to be done with thin cheaper wires, saving even more money.

Then the power transport in cables has losses along the line, electrons flow heats the cables, with high voltage, you can transport power by moving fewer electrons but with higher force in each of them. So again, another win for high voltage systems, and again high voltage production is easier if done in AC. There are DC high voltage lines, for example, submarine cables and nation to nation lines may be high voltage DC; not so common but it’s done sometime, for example, a country with 115v 50hz AC may transform it to DC to sell it to another country that then transforms it into its own 220V 60hz AC. This is done because the 2 different AC systems are completely incompatible, because both need to transform it, they may have a DC line between them.

Add in that illumination can be done with DC or AC without difference.

Basically the only drawback for AC is computers. Computers need low voltage DC. So we use an AC line for everything, and transform it to low voltage DC only in the devices that need it. Another thing that can’t work in AC are batteries, so any system that has a battery needs to be DC. This doesn’t mean that we can’t do a AC system with a DC conversion between system and battery, sometime it’s done, it’s overly expensive but may be done.

AC electricity has several important advantages.

The voltage used to transfer power is extremely important for energy efficiency and cost. The higher the voltage, the more energy efficient it is, and the thinner and cheaper the wires you need. For example, if you tried to heat a house with 12 V power, you would need cables as thick as your arm, and which would cost $100 per foot.

AC can have its voltage converted easily with low tech equipment using transformers. Transformers work because a changing electrical current causes a changing magnetic field, and a changing magnetic field can cause a changing electrical current. This allows a high voltage wire to transfer energy through a magnetic field into a low voltage wire. Note that the changing is important. If there is no change, there is no power transfer. Because AC constantly changes polarity, transformers provide a constant power transfer.

The use of transformers means that voltage can be increased to 500,000 volts for transfer between cities, then down to 50,000 volts for transfer in a city, then down to 10,000 for transfer between neighborhoods, then down to 100-400 volts for use by homes and businesses. Then electrical equipment can change the voltage down again to whatever they need.

The second advantage is that motors and generators are simpler and more reliable when built for AC power. In an AC motor, because the electrical voltage fluctuates constantly, it produces a constantly changing magnetic field. With some simple tricks, that fluctuating magnetic field can be made to look like it is rotating. If you have a rotating magnetic field, then it will drag a magnet around with it, and you have a motor. This means that an AC motor only has a rotating magnet and shaft as its moving parts. There is no need for electronics or complicated mechanisms.

A DC motor is different, because the magnetic field doesn’t fluctuate itself, it can’t make anything rotate on its own. So a DC motor uses a switch mechanism (called a commutator) attached to the shaft to convert the DC electricity into AC, which can then produce a rotating magnetic field. The switch mechanism contains moving and rubbing parts, so it wears out and requires regular maintenance. However, you can get motors with electronic commutators which use wireless sensors and electronic switches, these don’t wear out, they are electronic and quite high tech.

A similar issue occurs with generators. If you just spin a magnet next to some wire, you get AC power. If you want DC power, you have to convert it in some way.

When electricity for homes and businesses was being developed and rolled out, the obvious winner was AC. The generators were simpler (just rotating magnets), the transmission was simpler (transformers made long distance power lines cheap and practical), and motors for machines were simpler, cheaper and needed less maintenance. Lights and heaters worked just fine on AC and DC, so it made no difference for these.